Regensburg 2022 – scientific programme

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O: Fachverband Oberflächenphysik

O 69: Surface Reactions and Heterogeneous Catalysis 2

O 69.7: Talk

Thursday, September 8, 2022, 16:45–17:00, H4

Faster oxygen adatom diffusion in a more densely packed CO adlayer on Ru(0001): A high-speed STM and DFT study — •Hannah Illner¹, Sung Sakong², Axel Gross², and Joost Wintterlin¹ — ¹Ludwig-Maximilians-Universität München, Germany — ²Universität Ulm, Germany

Oxygen atoms on a Ru(0001) surface covered with 0.33 monolayers (ML) of coadsorbed CO travel through the CO layer by the so-called door opening mechanism. It is facilitated by structural fluctuations in the CO layer and leads to an enhanced diffusion constant [Henß et al., Science 363, 715 (2019)]. Expecting a lower diffusion constant on a more densely CO-covered surface we have investigated how this mechanism changes when the CO coverage is enhanced. The experiments were performed by means of a variable-temperature, high-speed STM. In the analyzed temperature range between 239 and 280 K the CO layer is disordered at a coverage of 0.47 ML. The obtained trajectories of the O atoms show that jumps occur in six equivalent directions with the same probability, in contrast to 0.33 ML, where three directions are preferred. Surprisingly, the Arrhenius plot of the hopping rates suggests a lower activation energy for the diffusion at the higher CO coverage than at 0.33 ML of CO. Density functional theory calculations suggest that at θ(CO) > 0.33 ML clusters form at which the CO density is locally enhanced, and that the configurations of the O atoms with respect to CO molecules are modified. However, the door opening mechanism is still efficient. A weaker binding of the O atoms in the dense CO layer effectively leads to a lower diffusion barrier.